In previous experiments we have induced chronic atherosclerosis in rabbits, closely resembling atherosclerosis in man, by means of the synergy of immunological arterial injury and lipid rich diets. It is the purpose of this investigation to determine whether modest decreases in serum cholesterol resulting from changes in diet like those recommended for man will halt progression or lead to regression of this chronic coronary and aortic atherosclerosis in rabbits. Furthermore, we propose to study lesion regression in the living animal by means of clinical tests such as angiography and electrocardiography and to correlate any changes in these findings with evidence of regression. Finally, we propose to study the fine structure, surface topography, and chemical composition of chronic atherosclerosis following reductions in serum cholesterol that are likely to be achieved in man.